1. Field of the Invention
The present invention relates to a developing device used for an image forming device which forms a toner image, such as a copying machine, a printer, a facsimile machine or a multifunctional machine which is a combination of two or more of these devices, especially to a technique for preventing spillage and scattering of a toner in a mono-component developing device. Moreover, the present invention also relates to an image forming device comprising a developing device.
2. Description of the Related Art
In an image forming device which forms a toner image such as a copying machine, a printer, a facsimile machine or a multifunctional machine which is a combination of two or more of these devices, normally, an electrostatic latent image is formed on an electrostatic latent image carrier such as a photoreceptor in an exposure process; this electrostatic latent image is developed to form a visible toner image in a development process; this visible toner image is transferred onto a recording medium such as recording paper in a transferring process, or is primarily transferred onto an intermediate transfer member such as an intermediate transfer belt temporarily and is secondary transferred onto the recording medium from this intermediate transfer member; and the toner image transferred onto the recording medium in such a manner is fixed on the recording medium in a fixing process.
In the development process, for example, so-called non-contact development is sometimes carried out, in which a drum photoreceptor which is an electrostatic latent image carrier and a developing roller which is a developer carrier are supported and spaced at a constant interval to form an electric field between the developing roller and the drum photoreceptor, and a toner which is a developer on the developing roller is caused to fly over the electrostatic latent image on the drum photoreceptor to visualize the latent image. In such a non-contact development, it has been important to accurately maintain the interval between the drum photoreceptor and the developing roller.
Accordingly, the methods which are currently employed include the followings: a method in which spacers such as rollers each having a radius larger than that of the developing roller by a predetermined clearance are supported at both ends of the developing roller, and thrusted against the outer periphery of the drum photoreceptor; and a method in which a contact member is provided on a box which supports the drum photoreceptor and is brought into contact with the outer peripheral surface of the developing roller.
In such an image forming device, in order to supply a toner which is the developer onto the developing roller and uniformly apply the toner onto the developing roller to form a thin layer thereon, a toner supply roller and a thin layer forming blade are brought into contact with the developing roller. Accordingly, the developing roller is pushed in the direction of the drum photoreceptor by the contacting pressures of the toner supply roller and thin layer forming blade, whereby warping occurs in the developing roller, which prevents maintaining the predetermined clearance. As a result, a difference in developed density is disadvantageously created between the central portion and both ends of the developing roller.
Measures for solving this problem include rendering the developing roller in the shape of an inverted crown (the shape which is concave at the central portion in the direction of the rotation shaft). For example, Japanese Unexamined Patent Publication No. H6-258933 discloses an image forming device comprising at least a developing unit and an image carrier, the developing unit comprising at least a developer carrier, a developing container which rotatably supports the developer carrier, a contact member which is in contact with the above developer carrier, and when deformation is produced in the above developer carrier by the contacting pressure of the above contact member, the central portion of the circumferential surface of the image carrier or the developer carrier is formed in a convex or concave shape so that the distance between the circumferential surface of the image carrier and the circumferential surface of the opposing developer carrier has a predetermined value throughout their entire areas.
According to the image forming device disclosed in this publication, the central portion of the developer carrier or image carrier has a convex or concave shape, and therefore when deformation is produced in the developer carrier by a contacting pressure P of the contact member which comes into contact with the developer carrier, the distance between the circumferential surface of the image carrier and that of the developer carrier can have a predetermined value throughout their entire areas.
In a developing roller having the shape of an inverted crown, When the starting point (initial point) of this shape of an inverted crown is present in a region in which an image is formed (in the region where a toner thin layer is present), a change in curvature at the starting point is large, and therefore the contacting state of the roller and the thin layer forming blade becomes unstable even if the thin layer forming blade is formed from an elastic material, which forms a gap therebetween. This problem not only occurs when an elastic rubber roller (a roller around the periphery of which an elastic rubber layer is provided as the core) is used as the developing roller, but, for example, becomes especially noticeable when a hollow aluminium blast roller (coreless hollow metal roller mainly composed of aluminium having unevenness formed on its outer peripheral surface by the shot blast method to improve its ability to carry toner) is employed. Such a gap may lead to spillage of the toner from the thin layer forming blade; scattering of the toner from a diselectrifying member which is sometimes in contact with the developing roller; and insufficient leakage prevention of the toner by the toner sealing member disposed for both ends of the developing roller.
However, in the above-mentioned Japanese Unexamined Patent Publication No. H6-258933, no consideration is paid to the fact that the contacting state of the thin layer forming blade and the developing roller becomes unstable because of such a shape of an inverted crown. Accordingly, in the image forming device disclosed in this publication, the toner may spill into the image forming device, scatter and leak, and thus may contaminate the inside of the image forming device.